A11705. Circles of Waiting
编程题
普及/提高-
知识点
题目描述
A chip was placed on a field with coordinate system onto point $(0,0)$ .
Every second the chip moves randomly. If the chip is currently at a point $(x,y)$ , after a second it moves to the point $(x-1,y)$ with probability $p_{1}$ , to the point $(x,y-1)$ with probability $p_{2}$ , to the point $(x+1,y)$ with probability $p_{3}$ and to the point $(x,y+1)$ with probability $p_{4}$ . It's guaranteed that $p_{1}+p_{2}+p_{3}+p_{4}=1$ . The moves are independent.
Find out the expected time after which chip will move away from origin at a distance greater than $R$ (i.e.  will be satisfied).
Every second the chip moves randomly. If the chip is currently at a point $(x,y)$ , after a second it moves to the point $(x-1,y)$ with probability $p_{1}$ , to the point $(x,y-1)$ with probability $p_{2}$ , to the point $(x+1,y)$ with probability $p_{3}$ and to the point $(x,y+1)$ with probability $p_{4}$ . It's guaranteed that $p_{1}+p_{2}+p_{3}+p_{4}=1$ . The moves are independent.
Find out the expected time after which chip will move away from origin at a distance greater than $R$ (i.e.  will be satisfied).
输入格式
First line contains five integers $R,a_{1},a_{2},a_{3}$ and $a_{4}$ ( $0<=R<=50,1<=a_{1},a_{2},a_{3},a_{4}<=1000$ ).
Probabilities $p_{i}$ can be calculated using formula .
Probabilities $p_{i}$ can be calculated using formula .
输出格式
It can be shown that answer for this problem is always a rational number of form , where .
Print $P·Q^{-1}$ modulo $10^{9}+7$ .
Print $P·Q^{-1}$ modulo $10^{9}+7$ .
输入输出样例
输入 #1
0 1 1 1 1
输出 #1
1
输入 #2
1 1 1 1 1
输出 #2
666666674
输入 #3
1 1 2 1 2
输出 #3
538461545
说明/提示
In the first example initially the chip is located at a distance $0$ from origin. In one second chip will move to distance $1$ is some direction, so distance to origin will become $1$ .
Answers to the second and the third tests:  and .
Answers to the second and the third tests:  and .